Implement support for custom fonts

Some fonts with custom styles can't be used in gimp (or any app which relies on Pango for font handling, e.g. Inkscape)

This can be worked around if the fonts are renamed in the FontConfig object.

Current Problems

• Exporting to pdf doesn't work (there is a way to make it work but it is still hacky)
• Not backward compatible with previously created xcf files (that is, files creates before the addition of this code)
• Some edge cases e.g. where the FULLNAME reported by FontConfig is the same for 2 different fonts with different weights or different font files

Demo

• The Problem

Amber Whiskey family doesn't work except for the Aged variant, and even that isn't reported correctly

• Expected Behavior

Every style is shown correctly and can be used